Rechargeable battery packs are used to provide the power for many different types of portable, electrically powered tools. The integration of the battery pack into this type of tool eliminates the need to provide the tool with a power cord that is connected to an external power source. One type of tool a rechargeable battery pack is used to power is the cordless surgical tool. The elimination of a cord for a surgical tool offers several benefits over corded surgical tools. Surgical personnel using this type of tool do not have to concern themselves with either sterilizing a cord so that it can brought into the sterile surgical field surrounding the patient or ensuring that, during surgery, an unsterilized cord is not inadvertently introduced into the surgical field. Moreover, the elimination of the cord results in the like elimination of the physical clutter and field-of-view blockage the cord brings to a surgical procedure.
A typical rechargeable battery pack includes a number of rechargeable, current storing cells. The cells of many battery packs are NiCd cells. These cells are connected together to form what is referred to as a cell cluster. The cell cluster is contained within a housing that forms the body of the battery pack. The housing has exposed contacts. The contacts serve as the conductive members through which current is stored in the cell cluster and drawn out of the cell cluster.
A typical rechargeable cell has a cylindrical outer case formed of conductive metal. This case forms the base of the cell as well as the side wall. The case also serves as the negative terminal, the ground terminal, for the cell. A disk-like piece of metal serves as both the head of the cell and the positive terminal. In order to hold the head in place, the metal forming the outer case is bent over to form an annular, inwardly directed lip that extends around the positive terminal. An insulating ring separates the case from the head to prevent these components from shorting out.
During assembly of a cell cluster, the cases of the cells are typically electrically connected to the positive terminals of the adjacent cells. In order to facilitate making these connections and to minimize the size of the battery pack, the individual cells are tightly packed together. Also, some cells are inverted relative to adjacent cells so that the positive terminal, the head, of one cell is adjacent the base of the case of the cell to which it is connected. This makes it possible to electrically connect the cells together in series by securing a strap of conductive material from the base of one cell to the positive terminal of the neighboring cell.
In order to make the above described cell cluster, additional assembly steps are required. First, it is necessary to provide an insulating sleeve around the sides of the individual cells. This insulation prevents short circuits from being established between the cells due to their abutment against each other. It is also necessary to provide a ring of insulation around the outer lip of each cell case that holds the complementary cell positive terminal in place. This insulating ring prevents the conductive strap that extends from the head of a cell from establishing a short circuit with the adjacent lip.
In battery packs not intended for sterilization, paper or cardboard is frequently formed into the insulating sleeves. Each sleeve is dimensioned to extend a slight distance above the top of the case over which it is placed. Then, the portion of the sleeve that extends above the case is bent inwardly to form the insulating ring around the upper lip of the case. In battery packs designed for sterilization, tubular sections of heat shrink plastic are fitted over the cells to serve as insulating sleeves. These sleeves are shrunk over the cells over which they are fitted so that the upper portions of each sleeve extends inwardly and serves as the outer insulating ring. Once the insulating sleeves are fitted over the cells, the conductive straps of metal are welded or otherwise electrically secured between the positive terminals of the cells and the negative terminals of the adjacent cells. These conductive straps, in addition to electrically connecting the cells, hold the cells together to form the cell cluster.
Often, the cells forming a cluster are secured together by a second means in addition to the conductive straps. One reason it is desirable to further secure the cells together is that it makes the cell cluster easy to handle during the subsequent steps of assembling the battery pack. Moreover, the second means for securing the cells together typically restricts the movement of the cells relative to each other to a greater extent than this movement is restricted by the conductive straps. Consequently, this second securement means minimizes the stress imposed on the conductive straps that would otherwise be present if the movement of the cells was left unrestricted. The minimization of this stress results in a like reduction in the likelihood that a conductive strap will separate from one of the cells to which the strap is attached. Such separation would, of course, render an assembled battery pack useless.
To date, different methods have been employed to provide a supplemental securement means for holding a cell cluster together. Some cell clusters are provided with paper or cardboard binders that are adhesively secured to the top and bottom of the cells forming the cluster. Unfortunately, there are some risks associated with employing these binders in battery packs intended to be sterilized through autoclaving. This is because should any moisture penetrate the battery pack housing, it could cause the paper forming the binder to break down. Over time, the binder would decay and simply no longer perform the function for which it was designed. Moreover, the moisture which permeates a paper binder could cause an electrical short to develop.
There have been some attempts to provide cell clusters designed for autoclaving with binders formed of silicone. These binders are formed by applying a semisolid paste to the bottom and top surfaces of a partially assembled cell cluster. Once the paste cures, it forms an adhesive binder that secures the individual cells forming the cluster together. This adhesive binder is capable of withstanding the rigors of autoclaving. However, while this paste-formed binder is useful, it is difficult to manufacture. As discussed above, it is formed out of a semi-solid material. Accordingly, at a facility at which these battery packs are manufactured, the extra process steps that are performed when working with semi-solid materials must be employed. These processes include the cleaning steps that need to be performed on the equipment used to store and discharge the semi-solid paste. The requirement of having to perform these additional steps when manufacturing a rechargeable battery pack invariably adds to the complexity and cost of the manufacturing process.